1. Field of the Invention
The invention relates to a liquid air electrode having superior discharge capacity when incorporated in a metal-air battery, and a metal-air battery provided with the liquid air electrode.
2. Description of Related Art
Metal-air batteries are rechargeable batteries that use an elemental metal or metal oxide for the negative electrode active material and oxygen for the positive electrode active material. Since oxygen used for the positive electrode active material is obtained from the air, and it is therefore not necessary to seal the positive electrode active material in the battery, metal-air batteries are theoretically able to realize greater capacity than secondary batteries using a solid positive electrode active material.
In a lithium-air battery, which is one type of metal-air battery, the reaction of the following formula (I) proceeds at the negative electrode during discharge.2Li→2Li++2e−  (I)Electrons generated in formula (I) perform work in an external load via an external circuit, after which they reach the air electrode. Lithium ions (Li+) generated in formula (I) migrate within an electrolyte retained between the negative electrode and the air electrode from the negative electrode side to the air electrode side by electroosmosis.
In addition, the reactions of the following formulas (II) and (III) proceed at the air electrode during discharge.2Li++O2+2e−→Li2O2  (II)2Li++½O2+2e−→Li2O  (III)The generated lithium peroxide (Li2O2) and lithium oxide (LiO2) accumulate on the air electrode as a solid. During charging, the reverse reaction of formula (I) proceeds at the negative electrode, while the reverse reactions of formulas (II) and (III) proceed at the air electrode, thereby causing metal lithium to be regenerated at the negative electrode and enabling redischarging.
Conventional metal-air batteries were typically provided with a layer configuration consisting of a metal solid negative electrode, electrolyte solution and solid air electrode. In this type of conventional metal-air battery, a solid composed of the reaction products of formulas (II) and (III) in the form of lithium peroxide (Li2O2) and lithium oxide (LiO2) accumulate on the air electrode, and as a result thereof, the air electrode becomes clogged and contact between the electrolyte solution and the air is interrupted, resulting in the problem of impairment of charging and discharging. In addition, there were restrictions on the electrolyte solution that can be used due to the nature of the battery. Moreover, countermeasures were also required against moisture and carbon dioxide gas entering primarily from the air electrode. It was also difficult to recycle battery materials. In view of these problems associated with conventional metal-air batteries, the following non-patent document discloses a technology for a lithium-air battery that attempts to resolve the problem of solid precipitation on the air electrode in particular. This technology attempts to prevent precipitation of lithium oxide (LiO2), which is the solid reaction product at the air electrode, by arranging an organic electrolyte solution between a lithium ion-conducting solid electrolyte and a negative electrode and arranging an aqueous electrolyte solution between the electrolyte and the air electrode.
See Zhou Haoshen and 1 other, “Development of a New-type Lithium-Air Battery with Large Capacity”, [Online], Feb. 24, 2009, National Institute of Advanced Industrial Science and Technology, search date: Aug. 17, 2010, Internet. (URL:http://www.aist.go.jp/aist_j/press_release/pr2009/pr20090224/pr20090224.html)
The reaction of formula (IV) is thought to proceed at the negative electrode during discharge in the lithium-air battery disclosed in this document.Li→Li++e−  (IV)According to formula (IV), metal lithium Li begins to dissolve in an organic electrolyte solution as lithium ions Li+ and electrons are supplied to a lead. The dissolved lithium ions Li+ migrate to the aqueous electrolyte solution on the air electrode side after passing through the solid electrolyte.
On the other hand, in this lithium-air battery, the reaction of formula (V) proceeds at the air electrode during discharge.O2+2H2O+4e−→4OH−  (V)According to formula (V), electrons are supplied from a lead, and oxygen in the air reacts with water on the surface of the air electrode resulting in the formation of hydroxide ions OH−. These hydroxide ions OH− encounter lithium ions Li+ generated in the above-mentioned formula (IV) in the aqueous electrolyte solution on the air electrode side resulting in the formation of water-soluble lithium hydroxide LiOH.
In the lithium-air battery disclosed in the above-mentioned non-patent document, the discharge reaction proceeds accompanied by consumption of water (H2O). Thus, although it is necessarily to preliminarily store a large amount of water in the battery in order to improve discharge capacity, this results in the shortcoming of a relative decrease in the energy density of the battery.